Zoom zoom

[neB]

Here's a stupid question:

Why aren't there zoom functions on telescopes?

My binos have a you beaut zoom knob on them which makes them great for finding small things, like .... astronomical objects.

The other night I was pushing my 4.5 inch F8 (almost) reflector, with a 3x Barlow and a 4mm eyepiece, trying to get a better look at Saturn and it was damn near impossible to get the thing in view. It didn't help that my finder scope wasn't too well aligned for that magnification.

But I thought, if only I had a zoom so I could get it in frame and then close in.

Your wisdom please.

[[1ponders] (Paul)]

There are plenty of zoom eyepieces around neB. Try https://www.bintelshop.com.au/welcome.htm go to the eyepiece section and type in "zoom"

[iceman (Mike)]

The magnification is determined by the focal length (FL) of your telescope, divided by the focal length of your eyepiece.

So with your scope, it looks like about 900mm FL.. using a 3x barlow as well as a 4mm eyepiece!? That's crazy magnification, like over 900x magnification!

Firstly, that's way too much magnification for that scope.. it's way too much for practically ANY scope. The 4mm eyepiece on it's own would be too much magnification for the majority of nights you'll try and use it.

So let's step back to how you would "zoom". Start with a 25mm eyepiece. With your 900mm FL, that will give you 36x magnification. Saturn will be small, but at least you'll be able to centre it in the FOV of your eyepiece.

Then "zoom in" by choosing a shorter focal length eyepiece, maybe a 15mm. That will give you 83x magnification. Saturn is still a bit small.. so zoom in again. Practically, you'll probably want to go no more than 200x magnification. That means a 5mm eyepiece in your scope. Saturn will be bigger now, but it will also be dimmer and subject to the seeing conditions.. so it will probably be blurry and out of focus and not really what you were expecting to see.

On topic regarding "zoom eyepieces", they do exist. You turn the eyepiece the zoom in and zoom out.. however they don't retain focus, you'll need to adjust your focus after zooming in or out. Some brands are regarded quite well, such as the TV 3-6mm zoom (I think).. There's others that zoom over a larger range, like 10-25mm or so, but i'm not sure on the optical quality of those.

Without a doubt, some sacrifice has to be made in a zoom eyepiece - it's most likely the optical quality or ability to correct coma etc. Go to a star party or observing meet and try one out if you can, see if a zoom EP is really what you're after.

Hope i've helped.

[rmcpb (Rob)]

neB, Mike's advice is sound and you would do better with a few good eyepieces that don't take your magnification over about 110x and a 2x barlow for when conditions allow the mag to be pushed to the theoritical max of your scope of 225x

That said, I spend most nights at below 150x with my 8" unless conditions are really good.

Cheers

[astro_nutt]

Hi neB, Like the others..barlow lenses and various sized eyepieces does for scope's what the zoom adjustment does for Bino's...and that's is a very good question..no such thing as a stupid one..
Cheers!!

[neB]

Thanks all - esp Iceman.

I'm glad I asked now.

I should point out that I did get a look at Saturn through that suicidal magnification (I did say I was pushing it - experimenting), but it was a tad discoloured - blue one side, red the other!! Took me about 30 minutes and I was sweating by the end of it - at 11pm.

Also ditched the barlow and used 25, 10 and 4mm eyepieces to good effect. Exactly as you specified Mike - you must be a mind reader. Also threw on a blue filter which gave a really sharp image.

Don't think I'll invest in a zoom eyepiece, but wouldn't mind trying one, one day. Save my pennies for other more important equipment - new scope etc.

[rmcpb (Rob)]

neB,

Don't forget that some good eyepieces are never wasted investments. They would be the easiest way to improve your views with your present scope and they can be used on your next scope when you get one.

If your present scope takes 1.25" eyepieces then that would be the way I would go in the short term. Take a look at the Buy and Sell forum for some good deals

Cheers